Early mouse embryos cultured in vitro will be used to study the interactions between the embryonic cells and various RNA- and DNA-tumor viruses: several types of human adenovirus, C-type RNA tumor viruses, SV40 and polyoma virus. In most cases, the embryos will be infected by exposure to viral suspensions in vitro at different stages of early development; in some cases, viruses and viral nucleic acids will be microinjected directly into the cytoplasms of one-cell and two-cell mouse eggs. The effects of viral infections will be followed both in short-range experiments with embryos developing in vitro and in long- term studies after transfer of the infected embryos into foster mothers to allow postimplantation development. In the short-range experiments the fine structural and biochemical features of embryo-virus interactions will be described and the effects of viral infection on the expression of endogenous virus-like particles (A- and C-type particles) will be noted. In the long-range studies an examination will be made of the normalcy of postimplantation development and differentiation and the types of developmental aberrations that may occur. The possibility of integration of exogenous viral genomes into the genetics make-up of the embryo will be investigated.